There is a desire on the part of vehicle manufacturers to increase the “electrification” of vehicle auxiliary loads by reducing the number of accessories that depend directly on the fueled-engine as a prime mover. Examples include power steering pumps, hydraulic drives, engine cooling fan, air conditioning compressor, oil and coolant pumps, and air compressors. An advantage of accessory electrification is reduced engine loading, which facilitates greater engine performance, increased flexibility in locating accessories, reduced fuel consumption, more efficient accessory operation, and reduced emissions.
A typical way to manage a large number of electrical loads in a vehicle is to provide an electrical system having a higher voltage, such as 24 volts, and providing taps for connecting lower-voltage accessories, such as 12 VDC accessories. For this purpose, two 12 volt batteries can be connected in series with a center tap. The accessories are connected to the “lower” battery, i.e., the battery connected between an electrical ground and the center tap.
One disadvantage of series-connected batteries in a vehicle is the predominance of 12 VDC accessories being connected to one or more taps of a higher-voltage battery supply. Since many of these accessories are often installed after the vehicle is placed in service, and sometimes by unskilled personnel, the vehicle manufacturer has no means to anticipate battery loading. As a result, individual batteries in the series of multiple batteries may be under- or over-charged. Another disadvantage of series-connected batteries is that the batteries may be mismatched due to differences in age, condition and design. This can also lead to under- or over-charging of individual batteries in the series.
Some vehicles may have several battery power supplies. For example, a vehicle may have a primary battery supply for powering the engine starter and a secondary battery supply for powering accessories. Each battery supply may be assembled from a plurality of batteries. The discharge and load characteristics can vary considerably between the primary and secondary battery supplies. For example, the primary battery supply is used to provide high current for a relatively short period of time to start the engine while the secondary battery supply is used to provide a smaller amount of current to the vehicle's accessories for a longer period of time. The types of batteries used in the primary and secondary battery supplies may also be different. For example, a primary battery supply may use flooded lead-acid batteries while the secondary battery supply may use deep cycle absorbed glass mat (“AGM”) lead acid batteries. Each type of battery can have differing charge requirements. There is a need for a way to tailor battery charging for each battery, in each battery system. There is a further need for a way to route power between the primary and secondary battery power supplies in the event that one supply is needed to charge or augment the other supply.